KenCroswell.com

Two New Galaxies Orbiting the Milky Way

By Ken Croswell

April 19, 2006

Images by Daniel Zucker et al./Institute of Astronomy, Cambridge.

Astronomers in Europe and America have discovered two new
galaxies orbiting the Milky Way. One, in the constellation
Bo÷tes, is the least luminous galaxy ever seen. The other, in
Canes Venatici, lies at the fringe of the Milky Way's empire of
satellites.

Both galaxies may help resolve the "missing satellite"
problem. Scientists believe giant galaxies, like the Milky Way,
formed long ago when dwarf galaxies smashed together. However,
this theory predicts the Milky Way should have hundreds of dwarf
satellites--but astronomers have found only about a dozen.

Now Daniel Zucker and Vasily Belokurov of the University of
Cambridge and their colleagues have spotted two new satellite
galaxies. By analyzing data from the Sloan Digital Sky Survey,
the astronomers noticed an excess of stars in Bo÷tes and Canes
Venatici. Both constellations neighbor Ursa Major, where last
year astronomers discovered another satellite galaxy orbiting the Milky Way.

The first false-color image here shows the Bo÷tes dwarf.
The colors indicate how concentrated or spread out the galaxy's
stars are from one another. White and red mark regions of the
galaxy that have the densest concentrations of stars, while
yellow and green denote areas of the galaxy where the stars are
more spread out from one another.

The second false-color image shows the Canes Venatici dwarf. It uses the same color scheme.

The astronomers say the Bo÷tes dwarf is about 200,000
light-years from Earth, a bit farther than the Magellanic Clouds, the
Milky Way's two brightest satellite galaxies. The Canes Venatici
dwarf is more remote, located at a distance of 730,000
light-years. That's comparable to the distance of the Milky Way's two
most distant satellite galaxies, Leo I and Leo II.

The Bo÷tes dwarf is the least luminous galaxy astronomers
have ever seen. Its absolute magnitude is -5.7, so it emits only
about a third as much light as Rigel, a star in Orion. Rigel,
the most luminous star within a thousand light-years of Earth,
has an absolute magnitude of -6.8.

Prior to this discovery, the least luminous galaxy ever seen
was the Ursa Major dwarf. Its absolute magnitude is -6.75, comparable to Rigel's.

Both the Bo÷tes and Canes Venatici galaxies are classified
as dwarf spheroidals--gas-poor galaxies whose stars are spread
out from one another. The colors and brightnesses of their stars
resemble those in ancient globular clusters, suggesting the new
galaxies are also ancient.

Both galaxies have low metallicities: their abundance of
heavy elements such as oxygen and iron is only about 1 percent
that of the Sun (see data table below).

Small galaxies are usually metal-poor, because stars create
heavy elements, and small galaxies don't have enough stars to
manufacture many metals. Furthermore, if a star explodes in a
small galaxy, the galaxy's gravity is so weak that it can't hold
on to the elements the explosion forges. Instead, they sail out
of the galaxy and into intergalactic space.

In describing the Canes Venatici dwarf, Zucker and his
colleagues write, "The discovery of such a faint galaxy in
proximity to the Milky Way strongly suggests that more such
objects remain to be found." If enough of these satellites
exist, they could resolve the "missing satellite" problem.

The astronomers have submitted the Bo÷tes and Canes Venatici
papers to Astrophysical Journal Letters.